5 Future Gravitational Lensing

Gravitational lensing is an exceptional field in astronomy in the sense that its occurence and many of its
features – e.g. multiple images, time delays, Einstein rings, quasar microlensing, galactic microlensing, weak
lensing – were predicted (long) before they were actually observed. Although “prediction” or predictability
is considered one of the important criteria of modern science, many (astro-)physical phenomena are too
complicated for a minute prediction (just think of the weather forecast). The reason why this
worked here is that gravitational lensing is a simple geometrical concept which easily allows
qualitative estimates and quantitative calculations. Extrapolating from these thoughts, it should be
possible to look forward in time once again and predict future applications of gravitational
lensing.

However, at any given time it requires very good intuition, some courage and maybe even a bit of
ingenuity to predict qualitatively new phenomena. It does not need much of either to envision that the
known lensing phenomena will become better, sharper, more. My predictions for the next decade in this
sense are humble and modest:

No doubt there will soon be more determinations of accurate time delays in multiply-imaged quasar
systems. If the models will get more precise as well, the value of the Hubble constant determined from
a number of lens systems will be accurate to a few percent or better and will probably turn out to be as
reliable as values obtained with any other method [205].

The frequencies, image separations, redshift distributions of multiply-imaged quasars and their lenses
will become a major tool in differentiating between different cosmological models. The Sloan Digital Sky
Survey, e.g., will discover a few hundred new lensed quasars with very well defined selection criteria, ideally
suited for that purpose. Another angle on the cosmological model and the values of and offer the
statistics of arcs. The number of high redshift galaxies seen as arcs depends crucially on the
number of rich galaxy clusters at intermediate redshifts. And since different cosmological models
predict very different formation redshifts for clusters, this promising road should be followed as
well [18].

The new facilities which become available now or in the near future in the infrared/sub-mm/mm
domain – like SCUBA, SIRTF, FIRST, IRAM – will open a completely new window in these
wavelength ranges, with supposedly most spectacular results in the arcs and cluster lensing
regime.

Quasar microlensing will provide information on the structure of the quasars and the intervening
clumped matter. With the new X-ray telescope AXAF with its high spatial resolution it will become
possible to obtain X-ray lightcurves which due to the presumably smaller emission region will produce
dramatic microlensing events in multiply-imaged quasars. Maybe we can “map” the hot spots of quasars
this way.

The largest number of lensing events in the near future will doubtlessly come from the “local”
microlensing experiments monitoring galactic bulge stars. The art of taking spectra of highly magnified
stars during microlensing events (as pioneered by [108]) will open up the fascinating possibility to
investigate the metallicity of bulge stars in detail or even resolve the stellar surfaces and study their
center-to-limb variations. In addition of being an excellent tool to study the structure of the Milky Way,
galactic microlensing will also provide unbiased statistics on the fraction of binary stars (within certain
relative distances). Extending the sensitivity to higher mass ratios between the binary components will
naturally lead to the detection of planets around stars (at distances of many kiloparsecs!).
Microlensing has the advantage compared to all other Earth-bound planet search techniques that it is
able to detect Earth-mass planets! It is also imaginable that before too long such microlensing
events could be detected directly by monitoring astrometrically the position of the star very
accurately [124].

In due course we should also know quantitatively how much dark compact objects contribute to the
mass of the halo of the Milky Way, and what their mass range is. The “pixel lensing” will probe
other lines of sight through the Galactic halo by exploring the Andromeda galaxy and other
nearby galaxies. This will provide information on the three-dimensional mass distribution of the
halo.

Weak lensing will be used to map not just the outskirts of massive galaxy clusters, but also to trace the
large scale structure by its effect on the background population of galaxies. If we find good ways to
discriminate between source galaxies at various redshifts, this way we can ultimately produce a
three-dimensional map of the matter in the universe (rather than a light map)! This will be an utmost
useful thing for the understanding of structure formation and evolution; as an aside we will determine the
matter content of the universe .

Some other possible applications of lensing will be: The black hole in the Galactic center affects
all sources that are near or behind the center. Mapping this effect will be a complementary
determination of the black hole mass and will help to study the dynamics near the black hole. The
redshift of the most distant object will be pushed beyond z = 6, and it is quite likely that it will
be magnified by lensing. The next generation of experiments to map the cosmic microwave
background will be sensititive enough to detect the gravitational lens signature of the matter “in
front”.

What about the not-so-predictable or not-so-easily-predictable future of lensing? Ultimately every object
in the sky is affected by (ever so slight) lensing effects: this is the not-yet-reached regime of ultra weak
lensing. I would like to conclude citing two remarks that Bill Press presented in his lensing outlook at
the IAU Symposium 173 in Melbourne (1995). He mentions that “gravitational lens effects
…are present along virtually every line of sight” [142]. In a not quite so serious extrapolation.
Press points out that more and more astronomers will (have to) deal with lensing in the next
decade, so that lensing will become an “ubiquitous observational technique” and hence – for
better or for worse: “Gravitational lensing may well disappear as a unique sub-specialty in
astronomy”.